tree: Use <stdio.h> for snprintf

<stdio.h> header is used for input/output operations (such as printf,
scanf, fopen, etc.). Although some input/output functions can manipulate
strings, they do not need to directly include <string.h> because they
are declared independently.

Change-Id: Ibe2a4ff6f68843a6d99cfdfe182cf2dd922802aa
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82665
Reviewed-by: Yidi Lin <yidilin@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/string.h

@@ -4,7 +4,6 @@
 #define STRING_H
 
 #include <stddef.h>
-#include <stdio.h> /* IWYU pragma: export */
 
 void *memcpy(void *dest, const void *src, size_t n);
 void *memmove(void *dest, const void *src, size_t n);